In general, in a color television receiver set, a color amplification system for amplifying a color signal comprising a carrier color signal and a color burst signal is provided with a color gain control circuit comprising a manual gain control wherein the color gain can be adjusted by a manipulatable voltage, and an automatic gain control wherein the color gain can be automatically adjusted by an ACC (automatic color control) signal.
However, in the conventional color gain control circuit, the manual and automatic gain controls are separately provided and, therefore, not only is the cost high with the increased number of component parts used in the color amplification system, but also a relatively large space is required for installation of the color gain control circuit when the color amplification system is fabricated into an integrated circuit (IC).
The present invention has been developed with a view to eliminate the above described disadvantages and has for its first object to provide a novel color gain control circuit wherein the manual and automatic gain controls are integrated by connecting them together in cascade fashion and which can, therefore, be easily fabricated into an integrated circuit with the reduced number of the component parts.
A second object of the present invention is to provide a color gain control circuit wherein the manual gain control is constituted completely by a differential circuit construction so that transistors forming each differential pair can be in a thermally balanced condition to minimize change in gain of the manual gain control relative to temperature.
A third object of the present invention is to provide a color gain control circuit wherein the ACC signal for the automatic gain control is constituted by an output from a comparator for comparing the respective amplitudes of output from a B-Y demodulator and an R-Y demodulator or a G-Y demodulator so that noises superimposed respectively on the outputs from the B-Y demodulator and the R-Y demodulator or G-Y demodulator in the comparator are counterbalanced to each other to avoid any possible reduction in S/N ratio of the ACC signal even when in a weak electrical field.
A fourth object of the present invention is to provide a color gain control circuit for a color television receiver set having a color killer system so designed as to avoid the appearance of color noises on a picture screen which would result from the stoppage of the manual gain control during the condition in which color synchronism is out of order at the time of receipt a color television broadcasting, in which color gain control circuit there is provided a killer disabling means for disabling a color killer signal in response to a pulse of a width including a color burst signal period and slightly larger than the color burst period so that, during at least the color burst period, the manual gain control can be operable to supply a color burst to a phase comparator in an automatic color synchronizing circuit.